Nonidealities in CO2 Electroreduction Mechanisms Revealed by Automation-Assisted Kinetic Analysis

In electrocatalysis, mechanistic analysis of reaction rate data often relies on the linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to 10 electrochemical cells, where each cell can have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We used this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms that are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.


1.
I agree that the automated reaction system is a good approach to enhance the efficiency of the kinetic, mechanistic studies, which typically demands large volume of and tedious data collection.One concern for the automated reaction is that, prior to each electrolysis, a single cell pan is purged with CO2 gas for ~ 5 min before potential is applied.Also, the electrolysis in each "cell" was conducted in a fairly short time.I wonder if these are sufficient time lengths to ensure a "steady state" CO2RR reaction.This is related to the credibility and reproducibility of the electrolysis.The authors can provide some experimental data to prove if the adopted protocol generates stable and reproducible results.

2.
The mechanistic demonstrations in Figure 2 and 3 are confusing and very difficult to follow.The nomenclatures of terms are poorly annotated in the main text and should be improved.In the SI, the authors provided kinetic models that "best" fit the experimental trend.The process makes sense to me, but considering the broad readership of ACS Central Science, I think the authors may expand the kinetic derivation with clear statement on the assumptions, step-by-step derivation of each rate law, so that the results could be more accessible to most readers.

Reviewer: 2
Comments to the Author This manuscript details the development and application of a robotic system for analyzing the kinetics of the CO2 electroreduction reaction (CO2RR).The robotic system enables the sequential and automatic collection of electrochemical rate data from up to ten cells.The analysis reveals non-ideal kinetic features such as non-linearity and voltage dependence in the apparent bicarbonate orders, facilitated by the robotic system.To fit the experimental data, a series of kinetic models are proposed.The development and employment of automated reaction systems are highly desirable, however, as a novel approach, its reliability needs to be verified.The authors showed the general correlation between reported rates and those determined with the automated system, however, the absolute rates of the latter were about one order of magnitude lower (Fig. S8).This origin of this stark difference needs to be identified.Based on the pictures and schematics of the setup in the main text and SI, there is no forced convection during the CO2RR.This could induce severe mass transport limitation of CO2, and causing the lower measured rates.Since kinetic analysis was applied on the measured rates, an implicit assumption was all rates were kinetically controlled.This point must be verified experimentally for this work to be publishable.
In the cell design of the robotic system, the potential interference of the crossover of the product of the counter electrode, e.g., oxygen, needs to be excluded.

2.
Figures 2d-g and Figure 3d,e illustrate the proposed model for data fitting, but the notations are confusing.It is suggested to label every node with specified species (presented in supporting information) to enhance readability and provide more chemical information.

3.
What is the major difficulty in obtaining a model with a significant F-test value?The models and anlysis methods presented in this manuscript are similar to the previous work ( ACS Catal. 2020, 10 (7), 4326-4336. ) , where F-test results were provided.

4.
If the difficulty mentioned in the third comment arises from a lack of data, rate data corresponding to concentrations in the range from 10-3 to 10-2 should be replenished.A key advantage of an automated reaction system is the ease of obtaining large amounts of reproducible data.However, reactivity data in this work do not appear to be more abundant than existing work without the automated system, and the experimental errors are also not smaller.Thus, kinetically rigorous data at more concentrations and narrower spread should be provided to increase the reliability of the kinetic analysis.

5.
Operating workflow and protocol are important for a robotic system.A flow chart should be provided to demonstrate how to make the full use of the 10 cells in the system.Encouragingly, the reviewers were excited about our work, finding it to be "of interest to a broad electrochemical research community".The reviewers agreed that "the development and employment of automated reaction systems are highly desirable" and found our study to be "a promising platform", that despite being proof-of-concept, paves the way for the development of automated systems that would "enable[e] large volume[s] of electrokinetic data collection that is not only beneficial to the study of CO2RR, but other electrochemical reactions".
The reviewers' criticisms largely focused on further demonstrating robustness of the kinetic data and improving the clarity of the mechanistic discussions.To address this feedback, we have included new experimental flow rate dependence data, additional analyses of rate stability, and revised mechanistic figures.Additionally, we have made significant edits to mechanistic explanations and derivations in both the main text and SI.Key revisions in the manuscript and SI are highlighted in yellow, and we are attaching a point-by-point response to the reviewers' comments.
Reviewer 1 suggested we provide further evidence that we were measuring steady-state rates with our protocol.To address this, we added a new stability analysis of our rate data.Additionally, Reviewer 1 suggested improving the clarity of the mechanistic discussion and adding derivations for the presented rate laws.To address this, we made significant edits, including two revised main text figures, two revised main text sections, and one new SI section.Reviewer 2's comments also largely focused on validating the robustness of the experimental approach.The reviewer expressed concerns about mass transport, anode-cathode crosstalk, and the lack of a quantitative model fit for the data.For the first concern, we performed a flow rate dependence, which is the canonical experimental test for external mass transport limitations.Unfortunately, we were not able to implement any changes to address the second and third concern, because the associated experimental burden is outside the scope of this initial Author's Response to Peer Review Comments: facilitating such a thoughtful review of our manuscript.My colleagues and I have taken into consideration the reviewers' comments to further improve the work and are resubmitting a revised manuscript.